1. Field of the Invention
The present invention relates to a photothermographic material and an image forming method. More specifically, the invention relates to a photothermographic material and an image forming method which exhibit high sensitivity and improved image storage stability.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis and in the field of films for graphic arts, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems” by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. All of the patents, patent publications, and non-patent literature cited in the specification are hereby expressly incorporated by reference herein. In particular, photothermographic materials generally have an image forming layer including a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region.
Photothermographic materials utilizing an organic silver salt have a great merit of containing all components necessary for image formation in the film in advance and being capable of forming images only by heating. However, on the other hand, photosensitive silver halide remains in the material, and as a result, light scattering and light absorption due to the silver halide grains causes turbidity of the film, whereby the film becomes opaque. In order to avoid the above defects, it is required that a grain size of the photosensitive silver halide grains is minimized and the coating amount thereof is reduced, and therefore, sensitivity of the material is limited. Moreover, after image formation, various chemical components necessary for forming an image remain as is in an unexposed portion, and reaction products remain in the portion where image forming reactions have occurred. These remaining chemical components and reaction products exert adverse influences on storage stability of the image, and thus further improvements in image storage stability are required.
On the other hand, attempts have also been made at applying the photothermographic material as photosensitive material for photographing. The photosensitive material for photographing as used herein means a photosensitive material on which images are recorded by a one-shot exposure through a camera, rather than by writing the image information by a scanning exposure with a laser beam or the like. Conventionally, photosensitive materials for photographing are generally known in the field of wet developing photosensitive materials, and include films for medical use such as direct or indirect radiography films, mammography films and the like, various kinds of photomechanical films used in printing, industrial recording films, films for photographing with general-purpose cameras, and the like. For example, an X-ray photothermographic material coated on both sides using a blue fluorescent intensifying screen described in Japanese Patent No. 3229344, a photothermographic material containing tabular silver iodobromide grains described in Japanese Patent Application Laid-Open (JP-A) No. 59-142539, and a photosensitive material for medical use in which tabular grains that have a high content of silver chloride and have a (100) major face are coated on both sides of a support, which is described in JP-A No. 10-282606, are known. Further, photothermographic materials coated on both sides are also described in JP-A Nos. 2000-227642, 2001-22027, 2001-109101, and 2002-90941.
However, even higher sensitivity is required for recording X-ray images so as to reduce an amount of radioactive radiation exposure with respect to the human body. Attempts to achieve high sensitivity by the conventional methods described above inevitably result in an increase in fog, and therefore, a method to attain high sensitivity while keeping the fog to a minimum is required.
Furthermore, in photothermographic materials comprising high sensitivity emulsions, it is difficult to maintain image storage stability in comparison with photothermographic materials comprising low sensitivity emulsions, and therefore, improvement is demanded.